1. Field of the Invention
The present invention relates to an apparatus for controlling the relative phase of a camshaft (cam angle) to a crankshaft in accordance with the operating conditions of an internal combustion engine thereby to control the valve operation (opening and/or closing) timing of an intake valve and an exhaust valve. More particularly, it relates to a valve timing control apparatus for an internal combustion engine that serves to prevent deteriorations in driveability, fuel consumption and exhaust emissions by reducing errors in the calculation of a cam angle based on a crank angle signal and a cam angle signal.
2. Description of the Related Art
Recently, in internal combustion engines (hereinafter also simply referred to as an engine) installed on motor vehicles or the like, regulation of harmful substances contained in the exhaust emissions discharged from the engines to the atmosphere is becoming severe from consideration of the environment, and hence it is demanded to reduce the harmful substances in the exhaust emissions.
In general, in order to reduce the harmful exhaust emissions, there have been known two methods, one of which is a method of reducing harmful gases exhausted directly from engines, and the other method is to postprocess the harmful exhaust emissions with a catalytic converter (hereinafter simply referred to as a “catalyst”) arranged on an exhaust pipe,
Since reactions for making the harmful gases harmless do not take place in this kind of catalyst until a certain temperature is reached, as is well-known, for instance, it is important that the temperature of the catalyst is raised to its activation temperature early or quickly even at the cold starting of the engine.
Today, in order to improve engine power or reduce exhaust emissions and fuel consumption, there have been adopted valve timing control apparatuses capable of changing the intake and exhaust valve opening and closing timings for each cylinder according to engine operating conditions.
In this kind of conventional apparatuses, variable means (actuators) for changing the relative positions of camshafts to a crankshaft of an engine are installed, and the crank angle position (i.e., the rotational position of the crankshaft) and the relative phases of the camshafts with respect to the crankshaft are detected with the reference position of the variable means being stored in memory, so that the relative phases of the camshafts are controlled in accordance with the engine operating conditions.
In the past, this type of valve timing control apparatus has been shown in Japanese Patent Application Laid-Open No. Hei 6-299876 for instance.
In the conventional apparatus disclosed in the above document, a cam angle changing means comprising an oil control valve (OCV) and an actuator is mounted on at least one of an intake camshaft and an exhaust camshaft so that a relative phase difference between the crank angle and the cam angle is learned at the time when the cam angle changing means is out of operation.
However, note that a crank angle sensor in the above-mentioned conventional apparatus generates, as a crank angle signal, only one pulse (corresponding to a crank angle position as a control reference) within a control stroke (i.e., intake, compression, explosion or exhaust stroke) for each cylinder of an internal combustion engine, and the relative phase of the cam angle to the crank angle is detected based on the crank angle signal and the cam angle signal.
In cases where the crank angle signal including one pulse per stroke is used, however, it is necessary to measure the periods of time between successive pulses of the crank angle signal so as to calculate the cam angle.
In addition, even in cases where the crank angle signal including two or more pulses per stroke is used, it is similarly necessary to measure the periods of time between successive pulses of the crank angle signal in order to detect the cam angle.
FIG. 8 is a block diagram in which a valve timing control apparatus of the general type for an internal combustion engine is shown in relation to peripheral parts of an engine 1.
In FIG. 8, air is supplied from an intake pipe 4 to the engine 1 through an air cleaner 2 and an airflow sensor 3.
The air cleaner 2 cleans the air to be sucked to the engine 1, and the airflow sensor 3 measures the amount of intake air supplied to the engine 1.
In the intake pipe 4, there are arranged a throttle valve 5, an idle speed control valve (hereinafter called “ISCV”) 6 and an injector 7.
The throttle valve 5 adjusts the amount of intake air passing through the intake pipe 4 to control the output power of the engine 1, and the ISCV 6 adjusts the intake air bypassing the throttle valve 5 so as to control the rotational speed or the number of revolutions per minute of the engine 1.
The injector 7 supplies an amount of fuel corresponding to the amount of intake air to the intake pipe 4.
A spark plug 8 is arranged in a combustion chamber of each cylinder of the engine 1 for generating a spark to fire an air fuel mixture within the combustion chamber.
A plurality of ignition coils 9 (though only one of them being illustrated) supply high voltage energy to corresponding spark plugs 8.
The exhaust pipe 10 discharges exhaust gas that is resulted from the combustion of the air fuel mixture in each combustion chamber of the engine 1.
In the exhaust pipe 10, there are arranged an oxygen sensor 11 for detecting the amount of residual oxygen in the exhaust gas and a catalytic converter 12.
The catalytic converter 12 contains therein a catalyst comprising a well-known three-way catalyst which is able to purify harmful gas components (THC, CO, NOx) in the exhaust gas at the same time.
A crank angle detection sensor plate 13 is caused to rotate integrally with a crankshaft (not shown) which is driven to rotate by the engine 1, and the sensor plate 13 of a disk-shaped configuration has a multitude of projections (not shown) formed on its circumference at intervals of a prescribed crank angle (for instance, 10° CA). Also, untoothed or lost teeth portions are formed on the circumference of the sensor plate 13 at crank angle positions corresponding to a reference position of each cylinder.
A crank angle sensor 14 is arranged in an opposed relation to the sensor plate 13, so that it generates an electrical signal (i.e., pulse of the crank angle signal) to detect the rotational position (crank angle) of the crankshaft when each projection on the sensor plate 13 crosses the crank angle sensor 14.
The engine 1 is provided with valves for controlling communication between the combustion chamber in each cylinder and the intake pipe 4 and the exhaust pipe 10, and the driving or operation timings (opening and closing timings) of each valve (i.e., intake valve and exhaust valve) are determined by camshafts to be described later which are driven to rotate at a speed of ½ of the rotational speed of the crankshaft.
Variable cam phase actuators 15, 16 individually change the intake and exhaust valve opening and closing timings.
Specifically, each of the actuators 15, 16 includes a retard angle hydraulic chamber and an advance angle hydraulic chamber (not shown), which are divided or separated from each other, for relatively changing the rotational position (rotational phase: cam angle) of the corresponding camshaft 15C or 16C with respect to the crankshaft.
Each of the cam angle sensors 17, 18 is arranged in an opposed relation with respect to a corresponding cam angle detection sensor plate (not shown) for generating a pulse signal (cam angle signal) to detect the cam angle of the corresponding camshaft by each projection formed on the circumference of the cam angle detection sensor plate, like the crank angle sensor 14.
Each pulse included in each cam angle signal functions as a cylinder identification signal and it is also used for detecting the cam angle of the corresponding camshaft changed by the corresponding cam angle changing means.
Oil control valves (hereinafter referred to as “OCVs”) 19, 20 together with an oil pump (not shown) constitute an oil pressure supply system for switchingly controlling the oil pressure supplied to the respective actuators 15, 16 to control the cam phases of the corresponding camshafts. Note that the oil pump is driven by the crankshaft to supply hydraulic oil to the actuators 15, 16 through the OCVs 19, 20, respectively.
An electronic control unit (hereinafter referred to as an ECU) 21 in the form of a microcomputer constitutes a control means for controlling the engine 1. Specifically, the ECU 21 controls the injector 7, the spark plugs 8 and the cam angle phases of the respective camshafts 15C, 16C in accordance with the engine operating conditions detected by various sensor means 3, 11, 14, 17 and 18.
In addition, though not illustrated herein, a throttle opening sensor is mounted on the throttle valve 5 for detecting the opening degree thereof (throttle opening), and a water temperature sensor is installed on engine 1 for detecting the temperature of engine cooling water. The throttle opening and the temperature of cooling water are input to the ECU 21 as information indicating the operating conditions of the engine 1 in addition to the above-mentioned various sensor information.
As shown in FIG. 8, the engine 1 with a variable valve operating timing (VVT) mechanism is provided with the actuators 15, 16 for changing the relative phase positions of the camshafts 15C, 16C with respect to the crankshaft.
Next, reference will be made to the general engine control operation according to the conventional valve timing control apparatus for an internal combustion engine shown in FIG. 8.
First of all, the airflow sensor 3 measures the amount of intake air sucked into the engine 1 and inputs it to the ECU 21 as detection information indicative of an operating condition of the engine 1.
The ECU 21 calculates the amount of fuel corresponding to the measured amount of intake air, drives the injector 7 to inject the amount of fuel thus calculated into the intake pipe 4, and drives the spark plugs 8 to fire the air fuel mixtures in the corresponding combustion chambers in the cylinders of the engine 1 at appropriate timings by controlling the current supply time durations and the current interruption timings of the ignition coils 9.
Moreover, the throttle valve 5 adjusts the amount of intake air supplied to the engine 1 thereby to control the output torque thereof.
The exhaust gas generated by combustion of the air fuel mixture in each cylinder of the engine 1 is exhausted to the ambient atmosphere through the exhaust pipe 10.
At this time, the catalytic converter 12 arranged on the exhaust pipe 10 purifies hydrocarbons (HC) (unburnt gas components), carbon monoxide (CO) and nitrogen oxides (NOx), all of which are harmful substances contained in the exhaust gas, into harmless substances such as CO2, H2O and the like, which are then exhausted to the ambient atmosphere.
Here, in order to draw out the maximum purification efficiency of the catalytic converter 12, the oxygen sensor 11 is installed on the exhaust pipe 10 to detect the amount of residual oxygen in the exhaust gas, which is input to the ECU 21.
As a result, the ECU 21 controls the amount of fuel injected from the injector 7 in a feedback manner so as to make the air fuel mixture before combustion to be at the stoichiometric air fuel ratio.
Further, the ECU 21 controls the actuators 15, 16 (VVT mechanisms) according to the operating conditions of the engine 1 so that the valve opening and closing timings for the intake and exhaust valves are properly changed.
FIG. 9 is a timing chart that shows the respective pulse waveforms of the crank angle signal and the cam angle signal.
In FIG. 9, crank angle positions are represented by angles before the respective compression top dead centers of cylinders #1-#4.
That is, B05 (BTDC 5°) indicates 5° before top dead center (TDC), and B75 indicates 75° before top dead center. Symbols #1-#4 represent cylinders that come to their compression top dead centers, respectively.
The crank angle sensor 14 generates, as a crank angle signal, a train of pulses at crank angles of a prescribed interval (10° CA).
Furthermore, the crank angle signal includes no-pulse generation portions (corresponding to the untoothed portions) in which no pulse is generated at prescribedcrank angle positions (e.g., B95 or B95 and B105) as shown in broken line pulse positions in FIG. 9.
On the other hand, each of the cam angle sensors 17, 18 generates, as the cam angle signal, pulses at prescribed crank angle positions (e.g., B135 or B135 and B100).
Here note that the output positions (crank angle positions) of the crank angle signal and the cam angle signals in FIG. 9 are shown as ideal designed values including no manufacturing error or the like.
The ECU 21 calculates a reference crank angle position (B75) based on an untoothed or lost teeth portion of the crank angle signal, and identifies the cylinders of the engine 1 based on the number of lost teeth (i.e., a loss of one tooth: one lost tooth only at B95, or a loss of two teeth: lost teeth at B95 and B105, respectively) between the successive reference positions of the crank angle signal and the number of pulses of the cam angle signal therebetween.
When the cam angles are shifted to an advance angle side under the action of the actuators 15, 16 that constitute the cam angle changing means, the output signals of the cam angle sensors 17, 18 are also shifted to an advance angle side.
If the operating range of each of the actuators 15, 16 is an angular interval of 50° CA, a pulse of the cam angle signal at the most advanced angle (see a lower row in FIG. 9) is generated at a crank angle position advanced by an angle of 50° CA from the most retarded angle position (see a middle row in FIG. 9).
Now, reference will be made to the cam angle detection operation of the conventional valve timing control apparatus for an internal combustion engine while referring to FIG. 9.
Using a crank angle position (B75) of the crank angle signal which becomes a reference for the calculation of the cam angle, the ECU 21 in FIG. 8 calculates an angle θc from a cam angle signal position (B135) to the crank angle position (B75), based on which cam angles corresponding to valve operating (opening and closing) timings are calculated.
At this time, in order to calculate the angle θc from the reference position (B75) of the crank angle signal to the pulse detection position (B135) of the cam angle signal, there is used the relation between a time interval between successive reference positions (B75) of the crank angle signal and a time duration Tc from each reference position (B75) of the crank angle signal to the pulse detection position (B135) of the cam angle signal.
FIG. 10 is an explanatory view indicating the time required for the crankshaft to rotate each constant crank angle of (10° CA) when the engine 1 is in the steady-state operation (e.g., running at a rotational speed of 1667 r/m). In FIG. 10, the axis of abscissa represents crank angle [deg CA] and the axis of ordinate represents time [ms].
In FIG. 10, for instance, 55 [deg CA] indicates the time required for rotation from B65 to B55 (an angle of 10° CA).
Moreover, the time required for the crankshaft to rotate by an angle of 10° CA becomes longer in the vicinity of 0 [deg CA] that is compression top dead center, owing to the compressive resistance of the intake air.
On the contrary, after compression top dead center, the time required for the crankshaft to rotate by 10° CA becomes shorter due to the torque generated by combustion of the air fuel mixture.
Even if the engine 1 is in the steady-state operation, there takes place a variation in the required time resembling a sine wave cycle in which a maximum value is reached in the vicinity of compression top dead center at angular intervals of 180 [deg CA], as shown in FIG. 10.
FIG. 11 is an explanatory view showing the time variation of FIG. 10 as a table.
As shown in FIG. 11, when the rotational speed of the engine 1 is 1667 [r/m], it takes a time of 18 [ms] for the engine 1 or the crankshaft to rotate by 180 [deg CA], and at this time the average time for the crankshaft rotation of 10 [deg CA] is 1 [ms].
In addition, the time required for the crankshaft to rotate by 60 [deg CA] from a pulse signal position (B135) of the cam angle signal to a reference position (B75) of the crank angle signal becomes 5.568 [ms] because of the periodic or cyclic change of the rotational speed of the engine 1 due to its compression and combustion.
Accordingly, in cases where the cam angle is calculated by using the cycle time as in the above-mentioned conventional apparatus, an angle θc′ from the crank angle position (B135) of the cam angle signal to the reference position (B75) of the crank angle signal is represented by the following expression (1).                                                                         θ                ⁢                                                                   ⁢                                  c                  ′                                            =                                                                    5.568                    ⁢                                                                                   [                    ms                    ]                                    /                                      18                    ⁢                                                                                   [                    ms                    ]                                                  ×                                  180                  ⁢                                                                           [                                      deg                    ⁢                                                                                   ⁢                    CA                                    ]                                                                                                        =                              55.68                ⁢                                                                   [                                  deg                  ⁢                                                                           ⁢                  CA                                ]                                                                        (        1        )            
Therefore, a measurement error Δθ between the calculated angle θc′ and the actual angle θc is represented by the following expression (2).                                                                         Δ                ⁢                                                                   ⁢                θ                            =                                                θ                  ⁢                                                                           ⁢                  c                                -                                  θ                  ⁢                                                                           ⁢                                      c                    ′                                                                                                                          =                                                60                  ⁢                                                                           [                                      deg                    ⁢                                                                                   ⁢                    CA                                    ]                                -                                  55.68                  ⁢                                                                           [                                      deg                    ⁢                                                                                   ⁢                    CA                                    ]                                                                                                        =                              4.32                ⁢                                                                   [                                  deg                  ⁢                                                                           ⁢                  CA                                ]                                                                        (        2        )            
With the conventional valve timing control apparatus for an internal combustion engine as described above, even when the internal combustion engine is in the steady-state operation, the angular speed of the engine varies depending on its respective strokes such as compression stroke, combustion stroke, etc., thus giving rise to the following problem. That is, the cam angle is calculated based on the time between successive reference signals of the crank angle sensor and the time between the crank angle signal and the cam angle signal, and hence the cam angle thus calculated involves an error that is caused by the influence of variations in the angular speed of the engine.
In addition, there arises another problem in that since the relation between the time interval of successive reference positions (B75) and the time Tc from each reference position (B75) of the crank angle signal to a position (B135) of the cam angle signal is used, there takes place a measurement error Δθ between the calculated angle θc′ and the actual angles θc, and a calculation error of the cam angle becomes greater particularly during acceleration or deceleration of the engine than during the steady-state operation thereof.